This invention relates to control systems, and particularly to control systems using phase locked loops to deliver pulses from a source to a load, such as a stepping motor, whose inertial characteristics may be too slow to respond to the rate at which input signals, such as pulses, are applied.
It is customary to drive stepping motors with driving pulses produced by a pulsing circuit. Stepping motors, in common with other motors, exhibit an inertia which is more difficult to overcome while starting than after the motor has begun to move. Hence, the motor may not be able to respond immediately to the pulses which arrive during its standstill condition. In most cases, some of the pulses effect the proper stepping operation and some of them are lost. If the relationship between the number of pusles and the movement of the motor is not critical, the loss of these pulses is unimportant. However, where the motor operation depends critically upon the number of pulses applied, such as when the motor is to set a machine tool at a particular position determined by the number of pulses, the loss of pulses affects the positioning accuracy adversely. In fact, the motor may refuse to start at all.
The aforementioned copending application, discloses a system which avoids the loss of initial pulses while the motor is brought up to speed. According to this application, the input pulses are applied to the motor through a phase locked loop while the input pulses are stored and compared with the stored and compared output pulses at the loop VCO. The loop filter is provided with a delay, corresponding to or slower than the inertial characteristics of the motor at standstill, and hence allows the VCO frequency to rise or fall only at a rate slower than that determined by the motor inertial characteristics at standstill. The input and output pulses of the loop continue to be stored and compared until the loop locks and thereafter. When the input pulses stop, the loop filter, by virtue of its delay, causes the VCO to continue producing the output pulses until the comparator notes that the number of output pulses equals the number of input pulses, and gates the output pulses off.
This system provides the desired accuracy even if the input pulses arrive at such a fast rate and in such a short burst that they end well before the loop ever locks and the motor ever reaches its operating speed. However, despite the accuracy, this system suffers from a disadvantage. If the burst of input signals is very short in comparison to the filter delay, the VCO frequency rises only for a short time and then produces output pulses at its mimimum rate until the number of output pulses equals the number of pulses in the input burst. The motor then only operates at a very slow speed.
An object of this invention is to overcome these difficulties.
Another object of the invention is to improve control systems of this type.